(1) Field of the Invention
This invention relates to an ink jet printing apparatus and more particularly to an improvement over an ink jet print head and support of the kind disclosed in related application Ser. No. 045,832 filed May 1, 1987, now U.S. Pat. No. 4,814,795, and assigned to the assignee of this invention and herein incorporated by reference. Print head holders of this type maintain a predetermined space relationship between the ink jet print head of the printing apparatus and the surface of materials such as cartons or the like that are to receive the ink printing or marking.
The ink jet print head and support of the invention are adapted to be incorporated into an ink jet printing apparatus of the kind that can be operated to apply ink markings to a surface such as on the side of a carton or container conveyed past the printing apparatus. The markings can be characters, symbols and bar codes. Ink jet printing apparatus of this type generally include one or more print heads for printing on one or more sides of the carton.
(2) Description of the Related Art
In conventional ink jet print heads employed in ink jet printing apparatus, each print head has a vertical array of orifice nozzles from which ink can be emitted under pressure in the form of droplets emerging from the print head for impact upon the material surface. The nozzles are connected through individual valves to an ink source that is maintained under pressure. A programmable controller regulates the operation of the valves to cause ink to flow through the valves to the nozzles and to be emitted from the nozzles according to a pre-selected pattern or patterns. In this way, the ink droplets form the desired symbol, character, or bar codes on the material surface.
Ink emitted from an ink jet nozzle must travel a small fraction of an inch to form an ink droplet. This ink droplet grows wider as the distance of travel from the nozzle to the printing surface increases. As the width of the droplet increases a larger dot will be printed upon the material surface impacted by the droplet. As a result, as the droplet gets wider, the outer edges of the printed dots lose precision, and the quality of the printing deteriorates.
In a typical installation, the surfaces to receive the printing move relative to the print head or print heads. For example, the surfaces may be defined as one or more sides of a package such as a carton or container, and there may be a plurality of such packages in a row on a conveyor that transports the packages successively past the print head or print heads. It is common to have one or more print heads on opposite sides of the conveyor so that printing can be done on opposite faces of the package simultaneously.
To guide the packages into position relative to the print head or print heads, guide rails are commonly provided on opposite sides of the conveyor. These guide rails define planes that are spaced a predetermined lateral distance relative to the face of the nozzle block, and the guide rails confine the packages between them as they are transported by the conveyor. However, packages of the same nominal size vary in width due to manufacturing tolerances and there are undulations in the faces of the packages. Therefore, the span between guide rails must be great enough to accommodate the widest of the packages as permitted by the size tolerances. Smaller packages within the tolerances may not contact the guide rails. Accordingly, although the guide rails hold the packages at nominal distances from the print head nozzles, those distances vary as different package surfaces pass the nozzles and are imprinted. Yet, regardless of package size and position, at the instant ink droplets are emitted from the nozzles, the span between the face of the nozzle block and an opposing substrate or surface must be optimal.
Efforts have been made to improve the quality of ink jet printing, resulting in improvements in the design of the nozzle such as by the incorporation of jewelled orifice nozzles, and resulting in improvements in the composition of the inks. Examples of the results of some of these efforts are set forth in U.S. Pat. No. 4,378,564.
In all of these applications to date, an optimum range of space between the ink jet nozzles on the print head and the surface of the material to receive the printing has existed and print quality has been limited by the tolerance in the spacing that was needed to accommodate the variations in sizes of the packages.
Means of reducing this tolerance have recently been developed such as that disclosed in the related application Ser. No. 045,832, now U.S. Pat. No. 4,814,795 incorporated herein by reference, where print heads of the jet printing apparatus are spring biased toward the surface to be printed to maintain a substantially constant spatial relation between the ink jet nozzles and the surface of the container to be printed. The disadvantages of this system are that it requires an additional, separate slide mount structure to support the print head. The slide mount structure comprises rods slidably received in bearings that support the print head for transverse movement toward the substrate or surface. The operative surfaces of the rods are exposed and likely to collect dust and dirt in normal operation of the slide mount. The collection of dust and dirt on the rods impedes the proper operation of the slide and negatively affects the quality of the printing on the surface of the materials conveyed past the print head. Additionally, it is possible that the rods could become bent during the operative life of the slide mount. A bend in one of the rods would also impede the proper operation of the slide and have a negative effect on print quality.